1. (a) Field of the Invention
The present invention relates to a semiconductor device having a projecting electrode and, more particularly, to a technique for fabricating projecting electrodes on a semiconductor device.
2. (b) Description of the Related Art
Examples of mounting techniques for a semiconductor device using projecting electrodes include a flip-chip bonding and a ball grid array (BGA) bonding. To mount a semiconductor chip on a printed circuit board by using a flip-chip bonding technique, for example, an array of ball electrodes made of gold or solder are first formed on the electrodes of the semiconductor chip, then thermally melted by using a thermal-pressing technique or reflow technique after the ball electrodes are disposed on the lands of the mounting board, followed by hardening by cooling.
FIG. 1 shows a flip-chip bonding technique, wherein a semiconductor chip 31 is mounted on a printed circuit board 33 by using a plurality of ball electrodes 32. The ball electrodes 32 are bonded to respective solder lands of interconnects in the printed circuit board 33.
Ball electrodes are also used for mounting onto a mounting board a so-called BGA semiconductor device fabricated by mounting a semiconductor chip on a printed circuit board, ceramic circuit board or polyimide film having an interconnect pattern, followed by sealing the semiconductor chip with a sealing resin. The BGA semiconductor device generally comprises a plurality of ball electrodes, called solder bumps, made of solder balls and arranged in a two-dimensional matrix on the bottom surface of the semiconductor chip. In this text, a semiconductor chip having ball electrodes used for a flip-chip mounting technique, as mentioned above, is also called a BGA semiconductor device or chip.
A BGA semiconductor device 31 mounted on a circuit board 33 using ball electrodes 32 sometimes suffers from cracks 35 such as shown in FIG. 1 in the bases of the ball electrodes 32 after a long time operation. The cracks 35 are considered to be caused by a large difference in the thermal expansion coefficient between the BGA semiconductor chip 31 and the mounting board 33 mounting the same.
Specifically, a semiconductor chip 31 formed on a silicon substrate has a thermal expansion coefficient considerably lower than that of the mounting board 33 made of a resin. The heat generated in the semiconductor chip 31 during operation thereof and by other electric components generates a difference in the thermal expansion (shown by the lengths of arrows in FIG. 1) between the semiconductor chip 31 and the mounting board 33. The difference in the thermal expansion causes a shear force in the ball electrodes 32 to generate cracks 35 therein.
The problem cracks can be solved to some extent by filling the space between the semiconductor chip 31 and the mounting board 33 with a resin as shown in FIG. 2. Patent Publication JP-A-4-219944 describes the structure of FIG. 2, wherein a thermosetting resin 36 is used for the filling. To obtain the structure of FIG. 2, it is proposed in the publication that the semiconductor chip 31 mounted on the circuit board 33 be slanted and the space therebetween be filled with a thermosetting resin 36 having a low viscosity by using a nozzle 37 and a capillary function of the space, as shown in FIG. 3. However, this process has a disadvantage in that a semiconductor chip 31 or board 33 having a defect therein found after the filling cannot be replaced by another equivalent semiconductor chip or board.
Patent Publication JP-A-8-236654 proposes that the bases of the solder bumps be coated with a resin to improve the reliability irrespective of the thermally sever condition such as a thermal impact test after the mounting. The proposed structure may be obtained by the steps of coating the entire surface of the solder bumps formed on a semiconductor chip with a resin and subsequent removing the resin at the tops of the solder bumps, or by the steps of masking the tops of the solder bumps with a resin film, filling the space between the bases of the solder bumps with a resin, curing the filled resin and removing the masking film from the tops of the solder bumps. This technique, however, also involves other problems in that the removal of the masking film effected as by surface treatment raises the costs of the resultant semiconductor device, and in that the solder bumps are often peeled off due to the moisture absorbed after the removal of the masking film and a rapid heating during the mounting.
If only the space between the bases of the solder bumps are filled with resin and the resin is cured, which is also possible, the tops of the solder bumps are often oxidized due to the high temperature ambient, which causes defects after the mounting. The problem oxidation of the solder bumps may be also involved in the process using the removal of the masking film from the tops of the solder bumps, as mentioned above, or during the storage of the semiconductor device having the solder bumps. In any case, the solder bumps generally suffer from the oxidation in the thermal process and should be carefully treated as such.